My son found a very interesting Rock this summer, a few feet into the St Croix river in Stillwater MN If that's very odd white marks on it. some of them seem kind of like numbers, symbols, triangles etc Now you may think, that's easy, anyone could have grave those or painted them. that's the odd part, you run your fingernails over most of them, and you cannot feel them. And I've tried to scrub it and it ain't paint. Anybody seen something like this before?